A myrmecophilic orb-weaver spider avoids plant-defending acacia ants behaviorally by sitting very still

Myrmecophily is uncommon in spiders and adaptations that allow spider infiltration of ant colonies are poorly studied. Here, we document a novel interaction between the orb-weaver spider, Eustala oblonga, and the ant-defended acacia, Acacia melanocerus, in central Panama. These spiders occupy webs at night, but spend most of the day crouched directly against the surface of the acacias amidst plant-defending acacia ants. Detailed behavioral observations indicated that the spiders generally occupied areas on the acacias patrolled more actively by ants, but were attacked only if the spiders moved, which happened very infrequently. We hypothesized, therefore, that the spiders avoid ant aggression behaviorally by being still and not reacting to encounters by patrolling ants. We tested this hypothesis experimentally by comparing ant responses to moving vs. immobilized E. oblonga spiders and moving vs. immobilized individuals of another plant-inhabiting, orb-weaver spider (Argiope argentata) not naturally found on ant-acacias. Consistent with the hypothesis, ants responded significantly more aggressively to moving spiders of both species than to immobilized spiders. Further, moving E. oblonga spiders utilized a particular method of escape in which they suspended themselves on a strand of silk until ant activity waned and then returned to the plant surface and crouched quietly without further agitating the ants. In contrast, moving A. argentata spiders attempted to outrun the ants, further agitating them until the spiders were killed or dropped to the ground. Our results suggest that E. oblonga may be able to inhabit ant-defended acacias essentially by hiding in plain sight.